Resonator-based magnetic field sensors can principally be divided in two categories, depending on the origin of the magnetic force used to generate or modify the movement of the resonator. In a first case, a current flowing through the resonator creates a Lorentz force in the presence of the magnetic field to be measured (termed measurand in the following). In a second case, the measurand passively generates a torque on a magnetic material fixed to or deposited on the resonator.
In the frame of magnetic field sensors fabricated with microelectromechanical system (MEMS) technologies, numerous resonating-structure types can be exploited in many different oscillation modes. Beams [V. Beroulle, Y. Bértrand, L. Latorre, P. Nouet, “Monolithic piezoresistve CMOS magnetic field sensors”, Sensors and Actuators A, 103, 23-42, 2003], torsional plates [Z. Kadar, A. Bossche, P. M. Sarro, J. R. Mollinger, “Magnetic-field measurements using an integrated resonant magnetic-field sensor”, Sensors and Actuators A, 70, pp 225-232, 1998], and suspended mass [H. Emmerich and M. Schöfthalter, “Magnetic field measurements with a novel surface micromachined magnetic-field sensor” IEEE Transactions on Electron Devices, 47, no. 5, pp. 972-977, 2000], [T. C. Leichlé, M. von Arx, S, Teiman, I. Zana, W. Ye, M. G. Allen, “A low-power resonant micromachined compass”, Journal of Micromechanics and Microengineering, 14, pp. 462-470, 2004] have been reported.
Devices based on the Lorentz force usually provide an amplitude output. Several implementations are possible. The simplest one consists in operating the resonator in an open-loop excitation as presented in FIG. 1. In this case, an oscillator preferably tuned at the resonance frequency of the mechanical structure is used to deliver an alternating excitation current on the resonator. Together with the measurand, this current creates a Lorentz force which brings the resonator in oscillation. The amplitude of oscillation, which is typically proportional to the measurand, constitutes the output of the sensor; see V. Beroulle above. The performances of these devices may be improved thanks to the additional implementation of zero-force feed-back loops as proposed by Z. Kadar, see above. The drawback of open-loop-excitation approaches is that the matching of the excitation-oscillator tuning with the resonance frequency of the moving structure may degrade with aging or environmental condition changes, leading to important variation of sensitivity. To prevent this, closed-loop excitations providing automatic resonance-frequency tracking instead of an external oscillator can be used (see FIG. 2). The idea is to actuate the resonator with a measurand-dependent Lorentz force in phase with its velocity, in order to diminish or compensate its damping. This kind of device requires their oscillation amplitude to be stabilized. This can be achieved with an automatic gain controller (the setting of which is the output) or using constant amplitude output for the electronics. In this later case, the output is the movement amplitude of the resonator. Such concepts have been exploited, for example, by Emmerich, see above.
Recently, a system proposing an additional measurand-independent closed-loop excitation, which lets the system oscillate even in the absence of magnetic field has been described in WO 2005 029 107. In this case, the measurand only modifies the oscillation amplitude. Such architecture enables to calibrate the offset of the sensor, and is said to provide a better resolution.
Resonator-based magnetic field sensors exploiting magnetic materials have for example been described in U.S. Pat. No. 6,429,652 B1. In this application, the resonator is actuated by a frequency-tunable, open-loop, measurand-independent excitation. The alternating torque generated by the interaction of the moving magnetic material and the measurand acts as an additional spring constant on the resonator. Since the resonance frequency of the resonator is a function of the spring constant, it is affected by the magnetic field. In this patent, the resonance frequency is determined by scanning the excitation frequencies and searching for the point where the oscillation is maximal.
Sensors using the Lorentz force have the advantage not to require magnetic materials, which simplify their fabrication. Moreover, they do not suffer from unwanted hysteresis or magnetic saturation effects and therefore offer considerable input ranges. Finally, thanks to the active nature of the principle, these sensors can be made independent of the measurand by switching off the excitation current. This feature can be useful for calibration purposes. On the other hand, sensors using magnetic materials enable low-power systems (passive measurand excitation), and frequency outputs, even though such a frequency output has not been presented in U.S. Pat. No. 6,429,652, cited above.
Similarily to digital signals, frequency/time signals (frequency, period, duty-cycle, phase shift, etc.) offer a significantly higher noise immunity than amplitude signals (voltage or current), and are therefore well suited for electrically noisy environments or for long transmission lines. Moreover, frequency outputs can easily achieve wide dynamic ranges which are not limited between the noise level and the supply voltage, as it is the case of amplitude outputs. Furthermore, the signal multiplexing and conditioning circuitry for frequency output devices is usually less challenging as this modulation is less sensitive to the quality of the electronics, e.g. the linearity does not directly affect the frequency, and less vulnerable to noise or crosstalk. Finally, the analog-to-digital conversion of frequency signals can be performed by simple pulse counting, and thus can be executed by microcontrollers without any additional interface circuitry, i.e. without A/D converters and the like. Conversion accuracies generally higher than for amplitude signals can be achieved, thanks to the better precision of frequency than of voltage references.